This invention relates to a process in which a ceramic powder is produced by forming an aqueous solution including precursors of the ceramic powder material, mixing to emulsify the aqueous solution in a water-immiscible fluid, and drying and separating the emulsion droplets.
Ceramic materials have been made by the thermal decomposition of metal organic resins. Such a method is described by M. Pechini in his U.S. Pat. No. 3,330,697 issued July 11, 1957, and assigned to the same assignee as is the present invention. Another such procedure is described in the paper by Hennings et al entitled "Thermal Decomposition of (BaTi) Citrates into Barium Titanates", Journal of Solid State Chemistry, Vol. 26, pages 329-338, 1978. These thermal methods provide highly homogenous bulk material but not powder.
Finely divided ceramic materials have been made by emulsifying an aqueous solution of a metal salt in a water-immiscible fluid. The water of the emulsion is then evaporated without boiling away a significant part of the water-immiscible fluid, and the emulsion is transformed into a metal-salt sol. Subsequently, the sol is caused to coagulate (flocculate) e.g., by heating and/or by the addition of a propanol, ethanol, or the like. The heating is also for simultaneously thermally decomposing the metal salt to produce a refractory powder.
Another method, dedicated to making alkali-earth-metal titanate powders, begins by forming alcoholates of each of the precursor metals, mixing the alcoholates and refluxing the mixture, adding water to the mixture to hydrolize the alkoxides and form a precipitate, separating the precipitate from the solution and drying the recovered titanate-precursor powder.
The later two processes, by which particles of metal compounds are separated by precipitation from a solution, are capable of producing fine powders, but inevitably some of the fine particles so produced agglomerate to form large particles.
It is also known to form an aqueous solution of the ceramic precursors mixed with corn syrup, or alternatively a sucrose solution, and heating until dry and charred. The resulting char is ignited in a furnace at 600.degree. C. provided with excess air to burn off the carbonaceous material. However, this elementary process produces extremely fine particles, e.g. 500 angstroms (0.05 microns) and smaller that tend to form fused-together agglomerates even after comminution. Furthermore, vastly greater amounts of char are used, e.g. the weight of carbon convertible to char is 4 to 5 times the weight of the ceramic product. This would lead to difficulty in a production version of this laboratory process to control the highly exothermic burning of so much carbon and prevent the sintering and fusing of the particles. Furthermore this process inherently produces extremely fine spherical particles of a few hundred angstroms diameter whereas it is well known that the finer the particles the more difficult it is to produce ceramic slips wherein the particles are not agglomerated and the slip rheology is usable. The preferred particle size range for making ceramic dielectric articles is about from 0.1 to 1 micron (1000 to 10,000 angstroms).
There is described in the patent U.S. Pat. No. 4,654,075 issued Mar. 31, 1987 to Cipollini, assigned to the same assignee as is the present invention, a method for making ceramic and metal powders including forming an emulsion of water-soluble powder precursors in a water-immiscible organic fluid, and heating in an inert atmosphere to form a char of the organics in the original emulsion and finally calcining in air to remove the carbon from the char and react the ceramic precursors in each particle to form a fine ceramic or metal powder. Powders made by this method have a well controlled particle size of narrow distribution related directly to the droplet sizes of the parent emulsion.
It is an object of this invention to provide a simpler method dedicated to making finely divided ceramic powders.
It is a further object of this invention to provide such a method wherein initially the powder product precursors are dissolved in an aqueous solution that is emulsified in a water immiscible organic liquid to establish the ultimate sizes of the final powder particles, wherein the average ceramic particle size is within the range of 0.1 to 1.0 micron.
It is yet a further object of this invention to provide such a method wherein the emulsion is subjected to a low temperature drying step to form a sludge of spherical particles that is capable of being heated in air to transiently char and then to drive off all remaining organic materials and to calcine the spherical particles.